The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
Known spark-ignition (SI) engines introduce an air/fuel mixture into each cylinder which is compressed in a compression stroke and ignited by a spark plug. Known compression ignition engines inject pressurized fuel into a combustion cylinder near top dead center (TDC) of the compression stroke which ignites upon injection. Combustion for both gasoline engines and diesel engines involves premixed or diffusion flames controlled by fluid mechanics.
SI engines can operate in a variety of different combustion modes, including a homogeneous SI combustion mode and a stratified-charge SI combustion mode. SI engines can be configured to operate in a homogeneous-charge compression-ignition (HCCI) combustion mode, also referred to as controlled auto-ignition combustion, under predetermined speed/load operating conditions. The HCCI combustion includes a distributed, flameless, auto-ignition combustion process that is controlled by oxidation chemistry. An engine operating in the HCCI combustion mode has a cylinder charge that is preferably homogeneous in composition, temperature, and residual exhaust gases at intake valve closing time. HCCI combustion is a distributed kinetically-controlled combustion process with the engine operating at a dilute air/fuel mixture, i.e., lean of a stoichiometric air/fuel point, with relatively low peak combustion temperatures, resulting in low NOx emissions. The homogeneous air/fuel mixture minimizes occurrences of rich zones that form smoke and particulate emissions.
In engine operation, the engine airflow is controlled by selectively adjusting position of the throttle valve and opening and closing of intake valves and exhaust valves. On engine systems so equipped, opening and closing of the intake valves and exhaust valves can be adjusted using a variable valve actuation system that includes variable cam phasing and a selectable multi-step valve lift, e.g., multiple-step cam lobes which provide two or more valve lift positions. In contrast to the throttle position change, the change in valve position of the multi-step valve lift mechanism is a discrete change, and not continuous.
When an engine operates in a HCCI combustion mode, the engine operates at a lean or stoichiometric air/fuel ratio operation with the throttle wide open to minimize engine pumping losses. The engine torque response in HCCI combustion mode is limited by the charge temperature and composition. Therefore, if a sudden change in operating conditions occurs, such as a sudden increase in requested torque, the response to the torque request is limited by slow valve timing and exhaust gas recirculation (EGR) response. When the engine is controlled responsive to the torque request, objectionable audible combustion noise, or ringing, may occur.
A compression-ignition engine operates at relatively high geometric compression ratios in a range of 15:1 and 22:1 and greater in particular embodiments. Higher compression ratios increase the thermal efficiency of the compression-ignited engine. The compression ignition engine operates by introducing unthrottled air into the combustion chamber, thereby increasing the efficiency by decreasing pumping losses. In a compression-ignited engine, the ignition timing is controlled by timing the injection of fuel, known as start of injection (SOI) into the combustion chamber near the end of the compression stroke when the trapped air within the combustion chamber is at or above an auto-ignition temperature for the fuel or start of combustion (SOC). The heat release of the combustion process causes an increase in in-cylinder pressure forcing the piston downward in the same manner as the spark-ignited engine.
One embodiment of a compression-ignition engine may include operating the engine in a premixed-charge compression-ignition (PCCI) combustion mode. The PCCI combustion mode incorporates a compression-ignition combustion system with high flow rates of cooled exhaust gas recirculation (EGR) and an early SOI timing. Combining a high EGR rate and an early SOI results in a long ignition delay period prior to SOC. The ignition delay period exceeds the fuel injection duration during PCCI combustion resulting in a premixed combustion event at the SOC. Adequate premixing of the fuel and air, along with a high EGR flow rate, reduces the formation of locally rich regions that contribute to particulate matter formation. The high EGR rate acts as a charge diluent that suppresses the temperature of combustion below that at which significant amounts of NOx are formed.
In an engine operating in either HCCI or PCCI combustion modes, transitioning between a low load to a high load can be complex. The engine control module must coordinate actuations of multiple devices in order to provide a non-objectionable combustion process. During a transition between low-to-high load requests, the changes in charge temperature and composition are limited by time-rate changes in cam phasing, valve timing, and EGR flow. Until the required charge temperature and composition is achieved, either incomplete combustion or ringing may occur, leading to torque disturbances and objectionable audible combustion noise, respectively.